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Java基础：一个100%会发生死锁的程序

2019年04月18日  | 移动技术网IT编程  | 我要评论

    多线程是java工程师进阶所必须掌握的一项技能，也是面试中绕不过的一个环节，而死锁又是多线程同步失败的经典案例，对于复杂的系统，死锁是很难通过代码层面来做静态检测和排查的，所以有的面试官会从反向出发，让你手写一个死锁程序。
    先来看一个网络上常见的死锁程序（可能存在问题）：

public class deadlocktest {

    private static object lock1 = new object();
    private static object lock2 = new object();

    public static void main(string[] args) {
        new thread(() -> {
            synchronized (lock1) {
                system.out.println("thread1 acquired lock1");
                try {
                    thread.sleep(1000);
                } catch (interruptedexception e) {
                    e.printstacktrace();
                }
                system.out.println("thread1 try to acquire lock2");
                synchronized (lock2) {
                    system.out.println("thread1 acquired lock2");
                }
            }
        }, "t1").start();

        new thread(() -> {
            synchronized (lock2) {
                system.out.println("thread2 acquired lock2");
                try {
                    thread.sleep(1000);
                } catch (interruptedexception e) {
                    e.printstacktrace();
                }
                system.out.println("thread2 try to acquire lock1");
                synchronized (lock1) {
                    system.out.println("thread2 acquired lock1");
                }
            }
        }, "t2").start();
        
        // 检测死锁
        checkdeadlock();
        system.out.println("main thread end");
    }

    public static void checkdeadlock() {
        threadmxbean mxbean = managementfactory.getthreadmxbean();
        scheduledexecutorservice scheduled = executors.newscheduledthreadpool(1);
        // 初始等待5秒，每隔10秒检测一次
        scheduled.scheduleatfixedrate(()->{
            long[] threadids = mxbean.finddeadlockedthreads();
            if (threadids != null) {
                system.out.println("检测到死锁线程：");
                threadinfo[] threadinfos = mxbean.getthreadinfo(threadids);
                for (threadinfo info : threadinfos) {
                    system.out.println(info.getthreadid() + "：" + info.getthreadname());
                }
            }
        }, 5l, 10l, timeunit.seconds);
    }
}

    上面这段程序在99.99%的情况下都会发生死锁，但是从理论的角度来讲，死锁并不是100%会发生的，比如：线程t1先启动并获取了锁lock1，在休眠的这1s的过程中，jvm并未发生线程调度（实际上基本不可能），t2未得到执行也未获取到锁lock2，这时候t1休眠结束并获取了锁lock2，那么这种情况下就不会发生死锁了。
    如何写一个100%会发生死锁的程序呢？直接上代码：

public class deadlocktest {

    private static object lock1 = new object();
    private static object lock2 = new object();

    private static volatile boolean flag1 = false;
    private static volatile boolean flag2 = false;

    public static void main(string[] args) {
        new thread(() -> {
            synchronized (lock1) {
                flag1 = true;
                system.out.println("thread1 acquired lock1");
                while (!flag2) {
                    // 无限循环，等待thread2获取到lock2后再继续往下执行（相比使用thread.sleep(1000)在理论上是100%会出现死锁）
                    thread.yield();
                }
                system.out.println("thread1 try to acquire lock2");
                synchronized (lock2) {
                    system.out.println("thread1 acquired lock2");
                }
            }
        }, "t1").start();

        new thread(() -> {
            synchronized (lock2) {
                flag2 = true;
                system.out.println("thread2 acquired lock2");
                while (!flag1) {
                    thread.yield();
                }
                system.out.println("thread2 try to acquire lock1");
                synchronized (lock1) {
                    system.out.println("thread2 acquired lock1");
                }
            }
        }, "t2").start();

        // 检测死锁
        checkdeadlock();
        system.out.println("main thread end");
    }

    public static void checkdeadlock() {
        threadmxbean mxbean = managementfactory.getthreadmxbean();
        scheduledexecutorservice scheduled = executors.newscheduledthreadpool(1);
        // 初始等待5秒，每隔10秒检测一次
        scheduled.scheduleatfixedrate(() -> {
            long[] threadids = mxbean.finddeadlockedthreads();
            if (threadids != null) {
                system.out.println("检测到死锁线程：");
                threadinfo[] threadinfos = mxbean.getthreadinfo(threadids);
                for (threadinfo info : threadinfos) {
                    system.out.println(info.getthreadid() + "：" + info.getthreadname());
                }
            }
        }, 5l, 10l, timeunit.seconds);
    }
}

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